The term "multi-fiber cross-coupler" refers to a cross-coupler including a plurality of single-core optical fibers, each of which has a core surrounded by cylindrical optical cladding.
In general, in a conventional single-core optical fiber, the core diameter lies in the range 7 microns to 10 microns, and the outside diameter of the optical cladding is 125 microns. Commonly, such a conventional single-core fiber is surrounded by at least one protective layer around its optical cladding. It is said to be "stripped" in the absence of any protective layer around its optical cladding.
The term "multi-core optical fiber" refers to an optical fiber including a plurality of "elementary" fibers, each of which also has a core surrounded by optical cladding. In each elementary fiber, the core also has a diameter lying in the approximate range 7 microns to 10 microns, but the optical cladding has a small outside diameter lying in the range 25 microns to 40 microns. The elementary fibers of a multi-core fiber are mechanically secured together in cylindrical cladding that has a diameter of 125 microns, while being optically independent from one another.
A multi-fiber optical cross-coupler is designed to provide optical coupling between its fibers.
An article entitled "Monolithic wavelength-flattened 1 .times.7 single-mode fused fiber couplers: theory, fabrication, and analysis" published in "Applied Optics", vol. 30, No. 6, Feb. 20, 1991 describes such a multi-fiber optical cross-coupler.
That known cross-coupler includes 7 stripped conventional single-core fibers assembled together so that they touch one another inside a capillary tube and optically coupled together locally inside the tube, one of the fibers being central, and the others being peripheral. The optical coupling is obtained by necking the tube containing the 7 fibers, i.e. by forming a bi-conical taper in said tube. The energy injected into the central fiber via a first one of its ends propagates along the central fiber only until it reaches the taper, where it is distributed between the 7 fibers, and it propagates along all 7 fibers from the taper.
The taper is formed by subjecting the tube to localized heating resulting in the optical cladding inside the tube fusing, and by drawing the resulting assembly.
The taper causes optical coupling to occur between the central fiber and each of the peripheral fibers, and it causes optical coupling to occur between adjacent peripheral optical fibers. The coupling depends on the degree of fusion of the optical cladding, on the peripheral and longitudinal dimensions of the taper, and on the corresponding geometrical deformations in the optical cladding of the fibers inside the tube. It is difficult to control these parameters so as to achieve the desired optical coupling between the fibers. For this purpose, the energy received at the second end of the fibers is monitored dynamically so as to detect the optical coupling obtained, and so as to stop heating and drawing when the optical coupling obtained is the desired optical coupling.